The present invention relates to fluid flow regulators and, in particular, it concerns flow regulators based on the principles of labyrinth flow-attenuation devices such as are used in irrigation applications.
Various types of flow attenuators and flow regulators are known for achieving slow release of a fluid from a relatively high pressure source, such as is required for drip irrigation emitters. The most common of these, referred to here as "labyrinth-type devices", achieve flow attenuation by providing a circuitous flow path along which kinetic energy is dissipated through turbulence.
Labyrinth-type devices have been adopted widely in drip irrigation applications for a number of reasons. The use of turbulent flow for flow attenuation allows the size of the passageways to be relatively large, thereby facilitating passage of dirt without blocking the labyrinth. The turbulence itself helps to prevent accumulation of dirt. And the devices may be mass produced from plastic by injection molding at very low cost.
Nevertheless, while labyrinth-type devices offer effective flow attenuation, they fail to provide flow regulation. In other words, the rate of fluid output remains highly dependent upon the supply pressure to the device. This results in uneven irrigation with lower regions and regions closer to the fluid supply often receiving more irrigation than higher regions and regions further from the supply.
To address this problem, a number of pressure-responsive flow regulating devices have been developed. These devices generally employ an elastomeric diaphragm which is exposed on one side to the fluid supply pressure. The other side of the diaphragm faces an outlet chamber having a centrally disposed outlet and which is supplied through a pressure-reducing flow path, typically a small labyrinth. Any increase in the supply pressure flexes the diaphragm so as to reduce the size of the flow path through the outlet, thereby regulating the outlet flow rate.
Flow regulating devices of this type, referred to herein as "diaphragm-type regulators", have been shown to provide substantially constant flow over a considerable range of supply pressures. The use of elastomeric diaphragms, however, is accompanied by various problems. Most notably, the elastomeric materials employed undergo relatively rapid aging such that their elasticity and other mechanical properties may vary significantly over a period of a year or even a few months. Since the low flow rates of drip irrigation require regulation of the outlet clearance on the scale of fractions of a millimeter, even small variations in the elastic properties of the diaphragm may cause pronounced changes in operation of the flow regulator. An additional shortcoming, even in new diaphragm-type regulators, results from the long response time (or "relaxation time") of the elastomeric material, i.e., the time taken to return to its previous shape or to adopt its new shape when the pressure changes suddenly. This time lag may result in significant flow variations at times of sudden variations in supply pressure such as when the fluid supply is switched on or off.
An alternative approach to achieve regulated flow using a labyrinth-type flow attenuator is proposed by U.S. Pat. Nos. 5,400,973, 5,609,303 and 5,829,685 to Cohen. In these devices, an elastomeric membrane is used as a displaceable wall which lies across the top of a set of baffles. The wall is displaced by variations in supply pressure to successively contact more or fewer baffles, thereby varying the operative length of the resulting labyrinth. Here too, the devices suffer from the aforementioned limitations due to aging of the elastomeric materials, and due to the materials' long response times.
A further shortcoming of all devices currently in use which employ a labyrinth for flow attenuation is the inability of the plastic materials used to provide lasting sharp edges. As mentioned earlier, the flow attenuation provided by a labyrinth results primarily from turbulence generated by the baffles. It is well known that turbulence is induced much more effectively by sharp-edged obstructions. However, the plastic materials used for conventional labyrinth devices cannot be produced with highly sharp edges, and they rapidly become further rounded by the effects of the fluid flow during use. The low efficiency of the resulting labyrinths requires the use of greatly extended flow paths to achieve a given degree of attenuation.
There is therefore a need for a flow regulator which would provide substantially constant output over a wide range of input pressures and which would avoid the limitations associated with use of elastomeric materials.